Signal adding and differentiating circuits are known in various versions. Reference is made, for example, to U.S. Pat. Nos. 4,440,498; 4,653,917; and 4,479,715. Such circuits are also used in the so-called "Sagnac-Interferrometer" employing a fiber optical gyro, whereby a 3.times.3-coupling device and two or three detectors are used and the output signals are produced as a function of the so-called gyro rate. Reference is further made to an article "Integrated Optics for Fiberoptical Sensors" published in the Trade Journal "Laser and Opto-Electronics", No. 21(I)/1989.
All known circuits of this type, however, have certain disadvantages as will be described below. Known circuits are not capable to permit in a simple manner a direct access to the difference signal and to the sum signal of the two input signal currents. Another drawback of conventional circuits of this type is seen in the fact that they require a substantial effort and expense for the analog-to-digital conversion that follows.
At small gyro-angular rates, the two detectors of a fiber optical gyro with a 3.times.3-coupler receives signals having signal values corresponding to 20 to 30% of the maximum signal. A precise resolution of small gyro rates thus makes it necessary to achieve a very precise measuring of small changes which are superimposed on a large or high d.c. signal component. Thus, when such signals are digitized, it is necessary that the entire signal is digitized with the precision with which the small signal component is determined, and which small signal component is generated by the gyro rotation. As a result, expensive and high resolution signal converters are required. Additionally, it is necessary to operate the photo-diodes practically in a short-circuit operation in order to evaluate the d.c. signal components because only in this manner it is possible to assure the stability of the zero point or, in other words, the linearity of the signal conversion.